Sulfur-Doped Binary Layered Metal Oxides Incorporated on Pomegranate Peel-Derived Activated Carbon for Removal of Heavy Metal Ions

In this study, a novel biomass adsorbent based on activated carbon incorporated with sulfur-based binary metal oxides layered nanoparticles (SML-AC), including sulfur (S2), manganese (Mn), and tin (Sn) oxide synthesized via the solvothermal method. The newly synthesized SML-AC was studied using FTIR, FESEM, EDX, and BET to determine its functional groups, surface morphology, and elemental composition. Hence, the BET was performed with an appropriate specific surface area for raw AC (356 m2·g−1) and modified AC-SML (195 m2·g−1). To prepare water samples for ICP-OES analysis, the suggested nanocomposite was used as an efficient adsorbent to remove lead (Pb2+), cadmium (Cd2+), chromium (Cr3+), and vanadium (V5+) from oil-rich regions. As the chemical structure of metal ions is influenced by solution pH, this parameter was considered experimentally, and pH 4, dosage 50 mg, and time 120 min were found to be the best with high capacity for all adsorbates. At different experimental conditions, the AC-SML provided a satisfactory adsorption capacity of 37.03–90.09 mg·g−1 for Cd2+, Pb2+, Cr3+, and V5+ ions. The adsorption experiment was explored, and the method was fitted with the Langmuir model (R2 = 0.99) as compared to the Freundlich model (R2 = 0.91). The kinetic models and free energy (<0.45 KJ·mol−1) parameters demonstrated that the adsorption rate is limited with pseudo-second order (R2 = 0.99) under the physical adsorption mechanism, respectively. Finally, the study demonstrated that the AC-SML nanocomposite is recyclable at least five times in the continuous adsorption–desorption of metal ions.


Introduction
Ions of heavy metals are frequently discovered in products generated by the oil industry. Vanadium (V 3+ ), lead (Pb 2+ ), chromium (Cr 3+ ), and cadmium (Cd 2+ ) are some examples of heavy metal ions that can be found [1]. Significant opportunities exist for these metal ions to enter groundwater, surface water, and drinking water because of a wide range of processes, including oil processing (extraction, shipping, and storage), mining, and other activities; as a result, the danger posed to the environment as well as to the health of humans is increased [2]. To prevent diseases and syndromes in humans, metabolic processes require trace amounts of heavy metals [3]. However, because heavy metals are metal-ligand layers with relative ease and (2) the basic ligand forms strong interactions with the inserted metal ions [12]. Since soft Lewis basic sulfides have a high affinity for soft Lewis acidic metal ions, the ions are strongly adsorbed by the sulfides [2].
Herein, highly mesoporous activated carbon made from pomegranate peels was chemically precipitated with sulfur-based metal-layered manganese and tin ions to boost its adsorption capacity and efficiency. According to the literature reviews, the diffusion and nonisotopic exchanger processes are likely to be responsible for the ability of metal cation-doped adsorbent to selectively extract cations from complex matrix [2]. Utilizing FTIR, SEM, EDX, and BET to characterize the SML-AC nanocomposite, newly synthesized materials were employed as an adsorbent to remove Pb 2+ , Cd 2+ , Cr 3+ , and V 5+ ions from oilfield water samples. Langmuir, Freundlich isotherm, and kinetic models were used to verify the experimental procedure and adsorption capacity.

FTIR Spectroscopy
Surface functional groups of pristine AC and AC-SML were considered with FTIR, as shown in Figure 1. According to the AC spectrum, the band at 3400 cm −1 corresponds to the O-H groups anchored onto the carbon material. The IR bands at 2905 cm −1 , 1712 cm −1, and 1450 cm −1 and attributed to C-H, C=O, and C-C/C=C stretching of AC functionals and the skeleton of the carbon framework. After incorporating sulfur-layered metal oxide into AC, the IR spectrum of AC-SML showed a new sharp peak at 1020 cm −1 and small peaks at around 500 cm −1 . It should be noted that the peaks that appeared in the wide range of 500-1050 cm −1 corresponded to the presence of metal-carbon (M-O-C) and sulfur-metal (M-S-M) linkage in the nanocomposite. This claim is confirmed by a previous study which demonstrated that the extensive bands 500-900 cm −1 are attributed to the metallic layered derived from the M-O stretching and O-M-O bonding [13,14]. Hence, the proposed characteristics of IR bands in both spectra of AC and AC-SML imply the successful incorporation of sulfur-metal layered into the activated carbon.

SEM Microscopy
The newly synthesized pristine AC and AC-SML surface morphology were investigated using FESEM. Figure 2a illustrates the micrograph of AC. It is the external surface of the activated carbon which is quite irregular and full of cavities due to alkaline activation. After the immobilization of sulfur-layered metal oxide onto AC (Figure 2b), the micrograph indicates the formation of circular and bulk substances over AC. This trend confirms the formation of AC-SML nanocomposite.

SEM Microscopy
The newly synthesized pristine AC and AC-SML surface morphology were investigated using FESEM. Figure 2a illustrates the micrograph of AC. It is the external surface of the activated carbon which is quite irregular and full of cavities due to alkaline activation. After the immobilization of sulfur-layered metal oxide onto AC (Figure 2b), the micrograph indicates the formation of circular and bulk substances over AC. This trend confirms the formation of AC-SML nanocomposite. . EDX spectrum and elemental composition for raw AC (c) and AC-SML nanocomposite (d).

EDX Spectroscopy
To verify the presence of the preferred elements on plain AC and AC-SML nanocomposite, the EDX technique was utilized. Figure 2c,d represents the EDX signals and weight percentage of AC and AC-SML nanocomposite elements. The elemental analysis demonstrates the presence of two main elements over plain AC: carbon (73.24%) and oxygen (26.76%). Hence, after incorporating metal-S-oxide nanoparticles onto the AC surface, the expected elements were observed such as C, O, S, Mn, and Sn, with a weight percentage of 67.91%, 23.53%, 5.25%, 1.10%, and 2.14%, respectively. The EDX additionally performed the good dispersion of sulfur and layered metal (Sn-S-Mn) oxides on the activated carbon.   . EDX spectrum and elemental composition for raw AC (c) and AC-SML nanocomposite (d).

EDX Spectroscopy
To verify the presence of the preferred elements on plain AC and AC-SML nanocomposite, the EDX technique was utilized. Figure 2c,d represents the EDX signals and weight percentage of AC and AC-SML nanocomposite elements. The elemental analysis demonstrates the presence of two main elements over plain AC: carbon (73.24%) and oxygen (26.76%). Hence, after incorporating metal-S-oxide nanoparticles onto the AC surface, the expected elements were observed such as C, O, S, Mn, and Sn, with a weight percentage of 67.91%, 23.53%, 5.25%, 1.10%, and 2.14%, respectively. The EDX additionally performed the good dispersion of sulfur and layered metal (Sn-S-Mn) oxides on the activated carbon.

BET Surface Area
Specific surface area is an imperative factor in adsorption chemistry since it directly reflects the sorption capacity. BET technique based on the N 2 adsorption-desorption process utilized to record the surface area of plain AC and AC-SML nanocomposite. The N 2 adsorption-desorption curve is shown in Figure 3 for AC (a) and AC-SML nanocomposite (b). The specific surface area values were obtained at 356 m 2 ·g −1 and 195 m 2 ·g −1 , respectively. The specific surface area is decreased after incorporating sulfur-based metal oxide into the AC. The high specific surface area value for plain AC is probably due to the porous structure, which also provided a higher pore diameter (3.38 nm) than the modi-fied nanocomposite (2.92 nm). The pore volume and diameter indicate that the prepared materials are mesoporous, which is appropriate for adsorption. reflects the sorption capacity. BET technique based on the N2 adsorption-desorption process utilized to record the surface area of plain AC and AC-SML nanocomposite. The N2 adsorption-desorption curve is shown in Figure 3 for AC (a) and AC-SML nanocomposite (b). The specific surface area values were obtained at 356 m 2 ·g −1 and 195 m 2 ·g −1 , respectively. The specific surface area is decreased after incorporating sulfur-based metal oxide into the AC. The high specific surface area value for plain AC is probably due to the porous structure, which also provided a higher pore diameter (3.38 nm) than the modified nanocomposite (2.92 nm). The pore volume and diameter indicate that the prepared materials are mesoporous, which is appropriate for adsorption.

Types of Materials
The influence of the types of adsorbent material (raw Ac and AC-SML) on adsorbent efficiency was studied in similar conditions (dosage of 100 mg, pH 4, and time 60 min). According to Figure 4, the raw AC shows high efficiency for Cd 2+ and Pb 2+ ions and low efficiency for Cr 3+ and V 5+ . This probably is due to the occupation of AC active sites with Cd 2+ and Pb 2+ ions, since it is rich in oxygenated functional groups. Moreover, after modification of AC with doping SML (Sn-S-Mn) nanoparticles, an increment in the adsorption percentage was observed for selected metal ions (Cd 2+ , Cr 3+ , Pb 2+ , and V 5+ ). This phenomenon can be associated with the enhancement of the electrostatic interaction between sulfur (S 2− ) and a selected metal cation. Hence, the trend ( Figure 4) reflects the synergetic effect of AC-SML nanocomposite for the uptake of Cd 2+ , Cr 3+ , Pb 2+ , and V 5+ cations.

Types of Materials
The influence of the types of adsorbent material (raw Ac and AC-SML) on adsorbent efficiency was studied in similar conditions (dosage of 100 mg, pH 4, and time 60 min). According to Figure 4, the raw AC shows high efficiency for Cd 2+ and Pb 2+ ions and low efficiency for Cr 3+ and V 5+ . This probably is due to the occupation of AC active sites with Cd 2+ and Pb 2+ ions, since it is rich in oxygenated functional groups. Moreover, after modification of AC with doping SML (Sn-S-Mn) nanoparticles, an increment in the adsorption percentage was observed for selected metal ions (Cd 2+ , Cr 3+ , Pb 2+ , and V 5+ ). This phenomenon can be associated with the enhancement of the electrostatic interaction between sulfur (S 2− ) and a selected metal cation. Hence, the trend ( Figure 4) reflects the synergetic effect of AC-SML nanocomposite for the uptake of Cd 2+ , Cr 3+ , Pb 2+ , and V 5+ cations.

Effect of Solution pH
The solution pH was investigated from 2 to 7 since metal ions are highly influenced by solution pH. In the survey literature, the selected metal ions are existing in different states, including cadmium: 2-8.5 (Cd 2+ ), 8.6-10.5 (Cd(OH) − ), 10.6-12 (Cd(OH) 2 ) aq [15]; chromium: 1-3 (Cr 3+ ), 3.1-6 (Cr(OH 2+ )), 6.1-8 (Cr(OH + ) 2 ), 8-2 (Cr(OH) 3 ) [16]; lead: 1-6.1 (Pb 2+ ), 6.2-8.5 (Pb(OH) − ), 8.6-11 (Pb(OH) 2 aq) [17], and vanadium: Removal efficiency is experimentally investigated for selected heavy metal ions and shown in Figure 5. As can be seen, Cd 2+ and Pb 2+ provided low efficiency at low pH, which can be defined through the undesired protonation of the adsorbent surface followed by electrostatic repulsion. It is also noteworthy that the proton ions (H + ) trade competition with metal cations to adsorb over nanocomposite. Due to electrostatic interaction and coordination, the favorable removal efficiency was obtained for Cd 2+ and Pb 2+ at pH 4 to 6. A slight decrease in efficiency at pH 6-7 is due to the formation of metal hydroxide (Pb(OH) − ). Chromium and vanadium showed different trends due to their neutralization in the presence of a hydrogen wealthy medium. The other reason that may additionally interact in such pH-dependent behavior is the specific constructions and geometries of the metal cations with pH, which increased the adsorption efficiency. However, pH 4 was the best experimental condition for all metal cations.

Effect of Adsorbent Dosage
The amount of adsorbent is vital for the quantitative removal of adsorbate in the adsorption process. This is described via the adsorbent dosage in the range of 10-150 mg, as shown in Figure 6. The removal profile reflected that the adsorption efficiency is increasing for all selected metal cations. However, the quantitative adsorption efficiency increased from 52% to 95% when sorbent material varied from 10 to 80 mg. After that, the efficiency slightly changed up to 150 mg. Hence, the efficiency expansion can be attributed to more adsorption sites on the adsorbent since the solid phase surface area increases exponentially with the regular degree of adsorbent dosage. Thus, 80 mg was selected for all metal cations with maximum adsorption efficiency.
ing for all selected metal cations. However, the quantitative adsorption efficiency increased from 52% to 95% when sorbent material varied from 10 to 80 mg. After that, the efficiency slightly changed up to 150 mg. Hence, the efficiency expansion can be attributed to more adsorption sites on the adsorbent since the solid phase surface area increases exponentially with the regular degree of adsorbent dosage. Thus, 80 mg was selected for all metal cations with maximum adsorption efficiency.

Effect of Time
Adsorption's contact time is another crucial parameter for the effective uptake of metal ions from an aqueous solution. The impact of contact time for the adsorption of the selected heavy metal cation was investigated in the exclusive times in the range of 5-240 min. Figure 7 indicates that the removal increased gradually from 25% to 80% for all metal cations by increasing the time up to 120 min, except vanadium with 96%. With additional increase in the contact time up to 240 min, adsorption efficiency no longer extends significantly for vanadium, but the adsorption efficiency of lead, cadmium, and chromium ions increases to >92%. The first stage (5-120 min) is performing the availability of the active site to uptake metal cations, and after that, the adsorption sites are saturated, and the system reaches equilibrium [18]. Finally, the 120 min was selected for the further procedure due to the high percent removal.

Effect of Time
Adsorption's contact time is another crucial parameter for the effective uptake of metal ions from an aqueous solution. The impact of contact time for the adsorption of the selected heavy metal cation was investigated in the exclusive times in the range of 5-240 min. Figure 7 indicates that the removal increased gradually from 25% to 80% for all metal cations by increasing the time up to 120 min, except vanadium with 96%. With additional increase in the contact time up to 240 min, adsorption efficiency no longer extends significantly for vanadium, but the adsorption efficiency of lead, cadmium, and chromium ions increases to >92%. The first stage (5-120 min) is performing the availability of the active site to uptake metal cations, and after that, the adsorption sites are saturated, and the system reaches equilibrium [18]. Finally, the 120 min was selected for the further procedure due to the high percent removal.

Adsorption Kinetics
The adsorption time limitation is performed with kinetic models of pseudo-first-order and pseudo-second-order models. The linear varieties of the proposed kinetic models can be estimated with Equations (1) and (2), respectively.
The parameters are the following: Qe (mg/g) is equilibrium adsorption capacity, Qt (mg/g) adsorption capability at any time t. The time constates of k1 (1/min) and k2 (g/mg/min) corresponds to the first-order and second-order models, respectively. Figure 8a,b reflects the linear plots of kinetic models, which are plotted Ln(Qe-Qt)

Adsorption Kinetics
The adsorption time limitation is performed with kinetic models of pseudo-first-order and pseudo-second-order models. The linear varieties of the proposed kinetic models can be estimated with Equations (1) and (2), respectively. The parameters are the following: Q e (mg/g) is equilibrium adsorption capacity, Q t (mg/g) adsorption capability at any time t. The time constates of k 1 (1/min) and k 2 (g/mg/min) corresponds to the first-order and second-order models, respectively. Figure 8a,b reflects the linear plots of kinetic models, which are plotted Ln(Q e −Q t ) versus time and t/Q t versus time, and the values of parameters are listed in Table 1. Based on the determination coefficient (R 2 ), the contact time was fitted with pseudo-second order (>0.99) as compared to pseudo-first order (0.88-0.96). In addition, Q e values (theory) of second-order models were in good agreement with experimental adsorption capacity. Therefore, the kinetic rate is limited with pseudo-second order, in which the adsorption process has been controlled with electron sharing and electrostatic interactions.

Adsorption Equilibrium and Isotherm Models
The impact of the initial concentration of selected heavy metal ions is experimented on to consider the equilibrium capability of AC-SML nanocomposite. For this, 10 mg of AC-SML was applied to adsorb the Cd 2+ , Cr 3+ , Pb 2+, and V 5+ with a concentration range of 5-300 mg·L −1 in 120 min contact time. The equilibrium isotherm (Q e vs. C e ) was recorded and plotted as shown in Figure 9. The isotherm graph shows the expected Qe increases from 3 mg·g −1 to 85 mg·g −1 via increasing the concentration of selected heavy metal ions until it reaches equilibrium. The isothermal evaluation revealed that the adsorption process follows the IUPAC regular pattern (Type II) [19]. This pattern described the adsorption process following a monolayer pattern.
Molecules 2022, 27, x FOR PEER REVIEW 10 of 16 AC-SML was applied to adsorb the Cd 2+ , Cr 3+ , Pb 2+, and V 5+ with a concentration range of 5-300 mg·L −1 in 120 min contact time. The equilibrium isotherm (Qe vs. Ce) was recorded and plotted as shown in Figure 9. The isotherm graph shows the expected Qe increases from 3 mg·g −1 to 85 mg·g −1 via increasing the concentration of selected heavy metal ions until it reaches equilibrium. The isothermal evaluation revealed that the adsorption process follows the IUPAC regular pattern (Type II) [19]. This pattern described the adsorption process following a monolayer pattern. Hence, isotherm models were utilized to explore the adsorption pattern and mechanism. Therefore, the well-known isotherm models, namely Langmuir, Freundlich, Dubinin-Radushkevich (DR), and free energy were used to describe the adsorption pattern and adsorption mechanism. Langmuir is attributing the monolayer adsorption onto the homogenous surface. Freundlich reflects the multilayer adsorption onto the heterogeneous surface. DR confirms the multilayer adsorption over either homogenous or heterogeneous surfaces. Free energy suggests the adsorption of machoism, physical or chemical. Hence, the proposed models' linear mechanism formed by the following Equations (3)-(5): ln Q ln K 1 n ln C , (4) ln Q ln Q K , RT ln 1 1/C , Hence, isotherm models were utilized to explore the adsorption pattern and mechanism. Therefore, the well-known isotherm models, namely Langmuir, Freundlich, Dubinin-Radushkevich (DR), and free energy were used to describe the adsorption pattern and adsorption mechanism. Langmuir is attributing the monolayer adsorption onto the homogenous surface. Freundlich reflects the multilayer adsorption onto the heterogeneous surface. DR confirms the multilayer adsorption over either homogenous or heterogeneous surfaces. Free energy suggests the adsorption of machoism, physical or chemical. Hence, the proposed models' linear mechanism formed by the following Equations (3)-(5): C e/Q e = C e/Q m + 1 /K L Q m (3) ln Q e = ln K F + ( 1 /n) ln C e (4) ln Q e = ln Q S − K ad ε 2 (5) In the equations, Q m (mg·g −1 ) is maximum adsorption capacity, K L (L·mg −1 ), K ad, and K F [(mg/g)(L/mg) 1/n ] correspond to Langmuir, DR, and Freundlich constants, respectively. C e (mg·L −1 ) equilibrium concentration of heavy metal ions and 1⁄n reflect the favorability of the adsorption process. Q s (mg·g −1 ) is the DR theoretical sorption capacity. Finally, E is sorption energy (Equation (7)), directly displaying the adsorption mechanism. The E values < 40 kJ/mol are the physisorption mechanism, and E > 80 kJ/mol is the chemisorption mechanism [20,21].
The corresponding linear models of every isotherm have been plotted (Figure 10a,b), and the parameters of every model were calculated in Table 2. Based on the values of R 2 , it used to be assumed that both Langmuir (R 2 > 0.92) and Freundlich (R 2 > 0.90) models observe to describe the adsorption of selected heavy metal ions onto the AC-SML adsorbent nanocomposite. Hence, adsorption can be either monolayer or multilayer, with a maximum adsorption capacity of 37.03 mg·g −1 to 90.09 mg·g −1 for all selected metal ions. Moreover, the appropriate values of R 2 of the DR model confirm the favourability of the multilayer pattern. The values of energy approved the physisorption mechanism for both monolayer and multilayer adsorption patterns for selected heavy metal ions over AC-SML nanocomposite.
Molecules 2022, 27, x FOR PEER REVIEW 11 of 16 sorption energy (Equation (7)), directly displaying the adsorption mechanism. The E values < 40 kJ/mol are the physisorption mechanism, and E > 80 kJ/mol is the chemisorption mechanism [20,21]. The corresponding linear models of every isotherm have been plotted (Figure 10 a,b), and the parameters of every model were calculated in Table 2. Based on the values of R 2 , it used to be assumed that both Langmuir (R 2 > 0.92) and Freundlich (R 2 > 0.90) models observe to describe the adsorption of selected heavy metal ions onto the AC-SML adsorbent nanocomposite. Hence, adsorption can be either monolayer or multilayer, with a maximum adsorption capacity of 37.03 mg·g −1 to 90.09 mg·g −1 for all selected metal ions. Moreover, the appropriate values of R 2 of the DR model confirm the favourability of the multilayer pattern. The values of energy approved the physisorption mechanism for both monolayer and multilayer adsorption patterns for selected heavy metal ions over AC-SML nanocomposite.

Mechanism Study
The proposed adsorption mechanism between the selected heavy metal ions and AC-SML is investigated, as graphically shown in Figure 11. Hence, based on the pH study, high efficiency was obtained in the pH ranges of 4-6. Therefore, the possible electrostatic interactions and coordination between Cd 2+ , Cr 3+ , Pb 2+, and V 5+ and functional groups (sulfur-metal) of the AC could be the reason for the high removal efficiency at pH 4-6. In addition, the lower removal efficiency at low pH (<4) may be because of the AC-SML's protonation of active sites as well as the competition of H + for adsorption, while at high pH > 7, the selected metal cations are present as hydroxide form (M(OH x ) − ), which is the prominent repulsion force interactive between the adsorbates and negative surface charge of adsorbent (AC-SML -).

Mechanism Study
The proposed adsorption mechanism between the selected heavy metal ions and AC SML is investigated, as graphically shown in Figure 11. Hence, based on the pH stud high efficiency was obtained in the pH ranges of 4-6. Therefore, the possible electrostat interactions and coordination between Cd 2+ , Cr 3+ , Pb 2+, and V 5+ and functional groups (su fur-metal) of the AC could be the reason for the high removal efficiency at pH 4-6. I addition, the lower removal efficiency at low pH (<4) may be because of the AC-SML protonation of active sites as well as the competition of H + for adsorption, while at hig pH > 7, the selected metal cations are present as hydroxide form (M(OHx) − ), which is th prominent repulsion force interactive between the adsorbates and negative surface charg of adsorbent (AC-SML -). Figure 11. Schematic of the proposed mechanism for heavy metal ion removal by AC-SML.

Regeneration
To study the regeneration, 80 mg of the AC-SML was positioned into a test tub including selected Cd 2+ , Cr 3+ , Pb 2+, and V 5+ . After each adsorption process, the adsorbate were desorbed using 5 mL (HCl, 0.1 M) after being shaken for 30 min. Then, launche heavy metal ions were measured with ICP-OES. Then, the AC-SML was washed with ex cess distilled water and reused for a further experimental process for repeated five ad sorption-desorption cycles. The adsorption efficiency was obtained 85%, 81%, 87%, 80% for Cd 2+ , Cr 3+ , Pb 2+ and V 5+ , respectively. This procedure indicated that the removal effe tivity of selected heavy metal ions does not reduce notably for up to five cycles. Thus, can be claimed that the AC-SML nanocomposite can be regenerated for five adsorption desorption cycles. Figure 11. Schematic of the proposed mechanism for heavy metal ion removal by AC-SML.

Regeneration
To study the regeneration, 80 mg of the AC-SML was positioned into a test tube, including selected Cd 2+ , Cr 3+ , Pb 2+, and V 5+ . After each adsorption process, the adsorbates were desorbed using 5 mL (HCl, 0.1 M) after being shaken for 30 min. Then, launched heavy metal ions were measured with ICP-OES. Then, the AC-SML was washed with excess distilled water and reused for a further experimental process for repeated five adsorption-desorption cycles. The adsorption efficiency was obtained 85%, 81%, 87%, 80% for Cd 2+ , Cr 3+ , Pb 2+ and V 5+ , respectively. This procedure indicated that the removal effectivity of selected heavy metal ions does not reduce notably for up to five cycles. Thus, it can be claimed that the AC-SML nanocomposite can be regenerated for five adsorptiondesorption cycles. Table 3 indicates that the freshly synthesized AC-SML nanocomposite could be noteworthy for removing Cd 2+ , Cr 3+ , Pb 2+, and V 5+ ions. Compared to other adsorbents, the AC-SML nanocomposite showed high adsorption capacity at pH 4. The adsorption process duration was relatively fast. The AC-SML's improved adsorption capacity was due to the doping of the sulfur metal layer oxides on the AC, enhancing interactions with heavy metal ions and resulting in significant removal.

Pretreatment and Synthesis AC
The nanosized pomegranate peel-activated carbon was prepared according to the previous study [9]. Briefly, the raw pomegranate peel (PG) was washed and dried in the laboratory under ambient temperature and dark place. The dried and clean pomegranate peel was ground into a fine powder and sieved properly (<150 µm). Then, lignans were extracted by combining 20 g of pomegranate peel powder with 6 g of NaOH in 200 mL of distilled water and stirring for 24 h. After being diluted and filtered through filter paper, the mixture was rinsed with excess distilled water and finally dried in an oven at a temperature of 85 • C. Next, the uniformly sized powder was carbonized in a furnace at 400 • C for 3 h (under an N 2 atmosphere) to produce biochar. Using a mass ratio of 5:1 (AC/NaOH), 120 mL of distilled water, and vigorously stirring at 120 • C for 2 h, we thoroughly combined the produced carbon with the NaOH solution. Following filtration of the solution, the isolated product was dried in an oven at 120 • C for 10 h. It was transferred to the furnace to finish the activation process and heated to 800 • C for 2 h. After being neutralized in 0.01 M HCl and excess distilled water, AC was dried in an oven at 90 • C for 24 h [2].

Synthesis of SML-AC
Subsequently, 1 g S 2 , 0.1 g SnCl 2 , and 0.1 g MnCl 2 were added to a solution containing 2 g of the dried AC in 150 mL of distilled water, and the combination was agitated at 45 • C for 3 h (pH 9). Afterward, the solid was separated from the mixture and heated in the furnace for 2 h at 650 • C. The product (SML-AC) was then rinsed with extra distilled water and dried in an oven at 85 • C for 18 h [23][24][25] (Figure 12). Molecules 2022, 27, x FOR PEER REVIEW 14 of 16 Figure 12. A schematic for SML-AC synthesis and removal procedure.

Adsorbent Characterization
A Bruker Equinox 55 spectrometer was used to conduct Fourier-transform infrared spectroscopy (FTIR) in the 400-4000 cm −1 wavenumber region to identify the functional groups of the newly synthesized material. The surface morphology and elemental structure of the AC and SML-AC were examined using a field emission scanning electron microscopy (FESEM) TESCAN MIRA3 equipped Energy Dispersive X-ray Analysis (EDX). Brunauer-Emmett-Teller (BET) determined the surface area and pore size.

Removal Procedure
First, 30 mL of the sample solution that contained 30 mg·L −1 of the target analytes mixed with 40 mg of the SML-AC adsorbent. After 30 min of orbital shaking on a shaker, the mixture was then centrifuged to separate the SML-AC from the mixture (4000 rpm, 6 min). Lastly, a syringe filter made of cellulose with a mesh size of 0.2 was used to filter 5 mL of the supernatant, which included the metal ions in residual concentration. This sample was immediately subjected to an inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis. In the adsorption trials, multiple critical parameters such as pH (ranging from 2 to 7), SML-AC quantity (ranging from 10 to 150 mg), and contact duration (ranging from 5 to 240 min) were explored and improved to produce acceptable results from the adsorbent. Required parameters were calculated below [1,26].
C1 and Ce are initial and residual concentrations of ions (mg·L −1 ), respectively. V is the initial sample volume (mL), qe is the equilibrium adsorption capacity (mg·g −1 ), and W represents the adsorbent dosage (mg).

Adsorbent Characterization
A Bruker Equinox 55 spectrometer was used to conduct Fourier-transform infrared spectroscopy (FTIR) in the 400-4000 cm −1 wavenumber region to identify the functional groups of the newly synthesized material. The surface morphology and elemental structure of the AC and SML-AC were examined using a field emission scanning electron microscopy (FESEM) TESCAN MIRA3 equipped Energy Dispersive X-ray Analysis (EDX). Brunauer-Emmett-Teller (BET) determined the surface area and pore size.

Removal Procedure
First, 30 mL of the sample solution that contained 30 mg·L −1 of the target analytes mixed with 40 mg of the SML-AC adsorbent. After 30 min of orbital shaking on a shaker, the mixture was then centrifuged to separate the SML-AC from the mixture (4000 rpm, 6 min). Lastly, a syringe filter made of cellulose with a mesh size of 0.2 was used to filter 5 mL of the supernatant, which included the metal ions in residual concentration. This sample was immediately subjected to an inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis. In the adsorption trials, multiple critical parameters such as pH (ranging from 2 to 7), SML-AC quantity (ranging from 10 to 150 mg), and contact duration (ranging from 5 to 240 min) were explored and improved to produce acceptable results from the adsorbent. Required parameters were calculated below [1,26].
Removal efficiency (R%) = C 1 − C e C 1 × 100 Adsorption capacity (q e ) = V m × (C 1 − C e ) C 1 and C e are initial and residual concentrations of ions (mg·L −1 ), respectively. V is the initial sample volume (mL), q e is the equilibrium adsorption capacity (mg·g −1 ), and W represents the adsorbent dosage (mg).

Conclusions
This study developed an efficient nano-sized adsorbent for removing heavy metal ions from aqueous solutions using a metal-sulfur-layered oxide immobilized on pomegranate peel-derived activated carbon. The nanocomposite was prepared and used for removing selected heavy metal ions (Cd 2+ , Cr 3+ , Pb 2+ , and V 5+ ) from the water sample at pH 4. The equilibrium isotherm models were used to explain the adsorption capacity, which obtained 37.03, 78.74, 35.21, and 90.09 mg·g −1 for Cd 2+ , Cr 3+ , Pb 2+ , and V 5+ , respectively. The Langmuir and Freundlich model and free Energy findings were carried out in the selected heavy metal ions adsorption process following monolayer and multilayer sorption patterns underneath the physical adsorption process. The kinetic rate was limited with pseudosecond order followed by electron sharing and electrostatic interactions. Hence, with a high removal efficiency of >90% for all selected heavy metal ions, the AC-SML nanocomposite is an efficient adsorbent for removing Cd 2+ , Cr 3+ , Pb 2+, and V 5+ from aqueous media.